Electric conductors and electric power cables incorporating them



Mud! 7 1970 D. R. EDWARDS ELECTRIC CONDUCTORS AND ELECTRIC POWER CABLESINCORPORATING THEM Filed May 21, 1968 ME bb Inventor DEREK [PEGIIVA LbEDWA/PIM A ttorney;

United States Patent 3,501,581 ELECTRIC CONDUCTORS AND ELECTRIC POWERCABLES INCORPORATING THEM Derek R. Edwards, Windsor, England, assignorto British Insulated Callenders Cables Limited, London, England FiledMay 21, 1968, Ser. No. 730,842 Claims priority, application GreatBritain, May 23, 1967, 23,897/67 Int. Cl. H01b 7/34 U.S. Cl. 17415 19Claims ABSTRACT OF THE DISCLOSURE This invention relates to electricconductors for the transmission of very large amounts of power andespecially to conductors for high voltage cables and high voltage cablesincorporating the conductors. The term cable is used herein to includeboth single and multiconductor cables and insulated busbars forming partof a busbar system for high voltage, high power, alternating or directcurrent transmission. The invention also relates to installations ofhigh voltage, high power cables.

The amount of power to be transmitted in a single high voltage, highpower cable circuit, that is by a single three-conductor cable or threesingle-core cables, constituting an AC. circuit, or by a two-conductorcable or two single-core cables constituting a DC. circuit, is upwardsof 1,000 megawatts and may be over 2,000 megawatts. Even at transmissionvoltages of 400 kv. AC. or 500 kv. DC, the cable losses in such circuitsembodying cables of conventional design may be about 200 kw. per cableper mile in the case of single core A.C. cables-operating at a conductortemperature of 85 C. and about 80% of that value in the case of eachcable of an equivalent D.C. circuit. In the case of an A.C. cable,almost 80% of this loss is the PR loss generated in the conductor. In aDC. cable the proportion is even higher. The generation of so much heatin such a cable presents difiiculties owing to the exceptionally thickwall of dielectric which makes it essential to provide some form offorced cooling of each cable conductor and to install heat exchangersfor the abstraction of heat from the conductor coolant. Thus apart fromthe initial high cost of such a cable due to a not inconsiderable extentto the presence of a heavy tonnage/ mile of cable of a high costcommodity, namely copper and/or aluminium, the installation is costlyalso because of the ancillary plant associated with the cable. The PRloss of itself constitutes a serious expense that is continuousthroughout the life of the cable-for such cables are generally used asvital feeders designed and intended to operate under full loadconditions at all times.

It is known that many metals at temperatures approaching absolute zero(0 Kelvin) undergo a change of state as regards their ability to conductelectrons, which is characterised by a disappearance of resistance toflow of a uni-directional current. This state of being able to conductelectrons without resistance, termed the superconductive state, isdestroyed by raising the temperature of the metal above a certain valuetermed the critical temperature. To date practical applications havebeen limited to metallic bodies which are superconductive above 4.2 K.,the normal boiling point of liquid helium. When the flow of currentthrough a superconductive material reaches a certain value the magneticfield which it generates reaches a critical value above which loss ofsuperconductiivty occurs. The critical value of this field depends uponthe particular metallic material and its temperature. Examples ofmaterials which become superconductive at temperature within a fewdegrees of absolute zero are aluminium, tin, lead, niobium and tantalum;niobium and certain alloys thereof are especially suitable for use inaccordance with the invention.

It has been proposed to take advantage of the phenomenon ofsuperconductivity of metals to design high voltage, high power cableswhich would have advantages over existing forms of force-cooled highvoltage, high power cables as regards compactness of design andtransmission losses. Metal which exhibit this phenomenon willhereinafter be referred to as superconductors.

The present invention is based on the discovery of an improved conductorespecially suitable for use in such cables and in accordance with theinvention the conductor comprises a composite bonded structure, of whichthe first layer is of a superconductor or superconductors, the secondlayer is of a metal or metals which are highly conductive when in anonsuperconductive state, and the third layer is of an alloy having botha very small thermal expansion coefiicient and a high mechanicalstrength such that the third layer has a high mechanical strengthrelative to each of the first and second layers.

The thickness of the first layer will normally be a small proportion ofthe thickness of the second and third layers.

The first layer, which is preferably a surface layer, must be of asuperconductor or superconductors of sufficient thickness to permit therated current of the cable to flow in the layer when in asuperconductive state with a low resultant dissipation in the layer,preferably not exceeding 10 microwatt/cm. of conductor surface. Niobiumof a thickness of 0.0025 cm. is suitable.

The function of the second layer is to safeguard the cable undertransient current overload conditions, when the superconductor is driveninto the normal state and ceases to be able to carry the current, andthe layer must be of sufficient thickness to perform this function andmust be in good contact with the first layer. For example when the highconductivity metal is annealed aluminium of 99.999% purity having anelectrical conductivity at 4.2 K. of at least 7 l0 (ohm-cm.)- or copperof a purity such that its conductivity is the same as that of 99.999%aluminium, a suitable thickness is between 0.05 and 0.1 cm.

The third layer has two functions namely to provide adequate mechanicalstrength to enable the conductor to be handled without damage and, whenin tubular form, to withstand a pressure differential of severalatmospheres, and to minimise the thermal contraction of the conductor oncooling to liquid helium temperatures, so that the necessity to resortto corrugation or the incorporation of bellows joints or other means ofaccommodating the change of length resulting from temperature change maybe avoided or minimised. The alloy or each alloy of which this layer ismade preferably exhibits an overall contraction, on cooling from ambienttemperature to the temperature at which the first layer becomessuperconductive, which is a small as possible and is preferably lessthan the strain at its yield point of the alloy. In

general, this will entail using an alloy or alloys having a coelficientof thermal expansion of less than 3x10 and preferably 2 X 10- cm./deg.K.

A preferred example of the third layer, suitable for use with theniobium and aluminium first and second layers specifically referred toabove, is a layer of 36% nickel-steel alloy of thickness 0.12 cm., suchan alloy being sold under the trade names Invar and Nilo as Invar 36 andNilo 36.

The composite conductor can be built-up as part of a composite structureincorporating another layer or other layers of conducting and/orinsulating material.

The cable in accordance with the invention may be of any suitable formprovided that it includes at least one composite conductor as specifiedabove and means for maintaining the first layer in its superconductivestate.

Preferred forms of cable comprise two or more co-axial conductorsarranged with their first layers facing each other. Coaxiality may bemaintained by spacers located helically or at frequent intervals alongthe length of the cable. Both conductors may be tubes of aniobium/copper/Invar or lead/copper/Invar or niobium/aluminium/ Invar orlead/aluminium/Invar laminate, the niobium or lead layer being on theoutside of the inner conductor and on the inside of the outer conductor.Superconductivity temperatures of the superconductive metal componentsof the composite conductors are obtained by maintaining a flow ofliquefied helium or super-critical gaseous helium through the annularpassage between the conductors, or within the inner conductor and aroundthe outer conductor. In the latter case, the space between the inner andouter conductors is maintained under high vacuum.

A example of the construction of composite conductors in accordance withthe invention suitable for use in cables in accordance with theinvention and two examples of such cables will hereinafter be describedby way of example with reference to the accompanying diagrammaticdrawing in which:

FIGURE 1 shows part of two concentric conductors arranged with anannular space between them,

FIGURE 2 is a cross-section of one form of cables, and

FIGURE 3 is a cross-section of another form of cable.

Referring to FIGURE 1, conductor 1 consists of a first (or surface)layer 3 of niobium of thickness 0.0025 cm.; an intermediate layer 4 ofannealed aluminium of 99.999% purity, thickness 0.1 cm.; and a third (orbacking) layer 5 of a 36% nickel-steel alloy of thickness 0.12 cm., thealloy being one that is sold under the trade names Invar and Nilo asInvar 36 and Nilo 36. The conductor 2 is formed from identical layers 6,7, 8 corresponding to the layers 3, 4, 5 respectively of conductor 1.

The annular space 9 between the conductors is evacuated and the annularspaces 10 and 11 immediately adjacent ot backing layers 5 and 8 arefilled with liquid helium or super-critical gaseous helium at atemperature below that at which the surface layers 3 and 6 of the twoconductors 1 and 2 are maintained in their superconductive state.

Alternatively the space 9 may contain liquid or supercritical gaseoushelium, when the spaces 10 and 11 will be evacuated.

FIGURE 2 represents a three phase cable having four concentricconductors 12, 13, 14 and 15, the inner conductor 12 carrying current ofone phase, the two intermediate conductors 13 and 14 both carryingcurrent of the second phase, and the outer conductor 15 carrying currentof the third phase. The conductors are identical with the conductors 1and 2 shown in FIGURE 1, the thicker line in FIGURE 2 representing thesuperconductor layer 3 or 6 and the thinner line representing the outerboundary of the backing layer 5 or 8.

The cable also incorporates inner and outer concentric metallic tubes 1and 17 which do not form part of the load carrying conductors of thecable. The tube 16, the annular space 18 between conductors 12 and 13,and the annular space 19 between the conductors 14 and 15 are allevacuated and the annular space 20 between the tube 16 and the innerconductor 12, the annular space 21 between the two intermediateconductors 13 and 14, and the annular space 22 between the outerconductor 15 and the outer tube 17 are all filled with liquid helium orsuper-critical gaseous helium.

Further layers of insulating material and/or evacuated spaces (notshown) are provided around the outer tube 17 as necessary to maintainthe helium at the desired temperature.

Referring to FIGURE 3 this is also a three phase cable having fourconcentric load carrying conductors, an inner tubular conductor 23, twointermediate conductors 24 and 25 and an outer conductor 26. As in thecable shown in FIGURE 2, the conductors are composite conductors of thekind shown in FIGURE 1 and are represented by a thick line indicatingthe position of the superconductor and a thin line indicating the outerboundary of the backing layer. In this cable the interior of the innertubular conductor 23 is evacuated, the annular space 27 between the twointermediate conductors 24 and 25 is evacuated, and an annular space 28between the outer conductor 26 and a metal tube 29 is evacuated. Theannular space between the inner conductor 23 and the inner intermediateconductor 25, the annular space between the outer intermediate conductor24 and the outer conductor 28, and the annular space between the tube 29and an outer metal tube 30 are filled with liquid helium orsuper-critical gaseous helium. Further layers of insulation and/orfurther annular evacuated spaces may be provided as necessary tomaintain the helium at the required temperature.

In both cables described by way of example, mechanical spacers (notshown) between the various concentric tubes forming the cable provideboth electrical and thermal insulation, except where adjacent layers areintended to be at the same electrical potential when the primaryfunction is to provide thermal insulation. In the cable described, withreference to FIGURE 3 the cooling fluid itself also functions as anelectrical insulating medium.

The sheath forming the external boundary of the outermost annulus of thecables described will be protected against mechanical damage byenclosing it in an outer casing from which it is preferably insulated bysuitable thermal insulation and/or by evacuating the clearance betweenit and the casing.

What I claim as my invention is:

1. An electric conductor for the transmission of very large amounts ofpower at high voltages when a layer thereof is in the superconductivestate comprising a composite bonded structure of which the first layeris of at least one superconductor, the second layer is of metal highlyconductive when in a non-superconductive state, and the third layer isof an alloy having a high mechanical strength relative to each of thefirst and second layers and a very small thermal expansion coefiicient.

2. A conductor as claimed in claim 1, in which the third layer is of analloy having both a thermal expansion coefficient of less than 2 10-cm./deg. K. and a mechanical strength such that on cooling from ambienttemperature to the temperature at which the first layer becomessuperconductive its overall contraction is less than the strain at itsyield point.

3. A conductor as claimed in claim 1, in which the third layer is of analloy of nickel and steel having a thermal expansion coefficient of lessthan 2X10 cm./ deg. K.

4. A conductor as claimed in claim 1 in which the third layer is of analloy of nickel and steel containing about 36% of nickel.

5. A conductor as claimed in claim 1 in which the sec. ond layer is ofannealed a uminium of 99.999% purity.

6. A conductor as claimed in claim 1 in which the second layer isannealed copper of a purity such that its electrical conductivity isequal to that of aluminium of 99.999% purity.

7. An electric cable for the transmission of very large amounts of powerat high voltages comprising:

(a) at least one conductor in the form of a composite bonded str-ucture,of which the first layer is of at least one superconductor, the secondlayer is of metal hi-ghly conductive when in a non-superconductivestate, and the third layer is of an alloy having a high mechanicalstrength relative to each of the first and second layers and a verysmall thermal expansion coefiicient, and

(b) means for maintaining the first layer in the superconductive state.

8. A cable as claimed in claim 7 in which the third layer of saidconductor is of an alloy having a thermal expansion coefiicient of lessthan 2 1()- cm./deg. K. and a mechanical strength such that on coolingfrom ambient temperature to the temperature at which the first layerbecomes superconductive its overall contraction is less than the strainat its yield point.

9. A cable as claimed in claim 7 in which the third layer of saidconductor is of an alloy of nickel and steel containing about 36% ofnickel.

10. A cable as claimed in claim 6 in which the second layer of saidconductor is of annealed aluminium of 99.999% purity.

11. A cable as claimed in claim 6 in which the second layer of saidconductor is of a purity such that its electrical conductivity equalsthat of aluminuim of 99.999% purity.

12. An electric cable for the transmission of very large amounts ofpower at high voltages comprising:

(a) at least two conductors, each in the form of a composite bondedstructure of which the first layer is of at least one superconductor,the second layer is of metal highly conductive when in anon-superconductive state, and the third layer is of an alloy having ahigh mechanical strength relative to each of the first and secondlayers, and a very small thermal expansion coeflicient, the conductorsbeing arranged coaxially with their first layers facing each other, and

(b) means for maintaining the first layer in the superconductive state.

13. A cable as claimed in claim 12, in which the annular space betweenthe two conductors is evacuated.

14. A cable as claimed in claim 12 in which the annular space betweenthe two conductors is filled with liquid helium.

15. A cable as claimed in claim 12 in which the annular space betweenthe two conductors is filled with supercritical gaseous helium,

16. A cable as claimed in claim 12 in which the third layer of saidconductor is of an alloy of nickel and steel containing about 36% ofnickel.

17 A cable as claimed in claim 12 in which the second layer of saidconductor is of annealed aluminium of 99.999% purity.

18. A cable as claimed in claim 12 in which the second layer of saidconductor is of a purity such that its electrical conductivity equalsthat of aluminium of 99.999% purity.

19. A cable as claimed in claim 12 in which the third layer of saidconductor is of an alloy having a thermal expansion coeificient of lessthan 2 10- cm./ deg. K and a mechanical strength such that on coolingfrom ambient temperature to the temperature at which the first layerbecomes superconductive its overall contraction is less than the strainat its yield point.

References Cited UNITED STATES PATENTS 3,331,041 7/1967 Bogner 335-2163,372,470 3/1968 Bindari- 29-599 3,428,925 2/1969 Bogner 335-2163,432,783 3/1969 Britton et al. 335-216 LEWIS H. MYERS, Primary ExaminerA. T. GRIMLEY, Assistant Examiner U.S. Cl. X.R.

